1. Field of the Invention
The present invention is directed to a method for reconstructing an image of a periodically moving object of the type employing a detector unit arranged on a carrier, the carrier rotating around a rotational axis with a rotational speed.
2. Description of the Prior Art
Measured datasets of a heart are conventionally registered with a detector unit having a single detector line that is arranged at a right angle relative to the rotational axis. During rotation of the carrier, measured datasets are registered by the detector line at a number of rotational angles. At the same time, a ECG signal is registered. Rotational angles are allocated to heart phases by means of the ECG signal. The heart exhibits a moving phase and an idle phase in each period. Image reconstructions are undertaken using registered measured datasets representing the idle phases. Subsequently, the patient is shifted a small distance parallel to the rotational axis and new measured datasets are registered.
Given standard rotational speeds (maximum of 120 revolutions/minute), it is not possible to tomographically image the complete human heart during the time span for which a patient can hold his or her breath, i.e. during a breath-holding pause.
It is also known to continuously shift the patient during the rotation of the carrier (what is referred to as a spiral scan) and to likewise continuously register measured datasets (referred to as spiral data), however, limits are quickly encountered. Not all shift or slice positions can be reconstructed from measured data registered during the idle phase of the heart. In the interpolation of the spiral data based on a predetermined shift or slice position, it is therefore necessary to interpolate using only data obtained during spiral revolutions that took place in the resting (idle) phase of the heart. This can substantially increase the interpolation width (i.e., the spacing between measured datasets which are used to produce an interpolated dataset therebetween) and consequently significantly reduce the image sharpness that can be achieved.
It is fundamentally possible to reconstruct an image of the object at all shift positions, however, only images which are qualitatively poor can be achieved at the shift position during which the heart was in a beating (moving) phase.
An object of the present invention is to provide an image reconstruction method with which qualitatively high-grade images of periodically moving objects can be registered and reconstructed with high time resolution, even with a conventional computed tomography apparatus.
For a subject exhibiting periodic motion with a motion phase and a resting phase, the object is achieved in a method wherein data are obtained from a detector unit having a first detector line and a last detector line, the detector lines proceeding substantially at a right angle relative to the rotational axis, wherein the presence of resting phases is identified and respective measured datasets respectively allocated to the rotational angles are registered at least during the resting phases at a number of rotational angles per rotational angle by each of the detector lines simultaneously, wherein the duration of the resting phase is identified and the rotational speed is selected such that the carrier rotates during a resting phase by a rotational angle that is at least as large as the reconstruction angle range required for the reconstruction of the image of the object, and wherein an image of the object is reconstructed from the measured datasets with a three-dimensional back-projection algorithm.
In this embodiment, measured datasets can be registered during the resting phases in such a large, continuous rotational angle range that a reconstruction of the object is possible, and the use of the three-dimensional back-projection algorithm substantially enhances the quality of the reconstructed image. The use of a multi-line detector unit considerably shortens the registration time. By combining the measures of a multi-line detector unit, registration of the measured datasets in the resting phases, suitable selection of the rotational speed, and use of a three-dimensional back-projection algorithm, qualitatively high-grade images of the object thus can be registered and reconstructed.
This image reconstruction method embodiment is preferably utilized for imaging the human heart. For determining the presence of a resting phase and the duration of the resting phase of the human heart, an electrocardiogram of the human is thereby preferably monitored.
When the periodic motion of the subject, by contrast, exhibits no resting phase or only a short resting phase or registrations of the subject should ensue during the motion phase, the object of the invention is achieved in a method wherein data are obtained from a detector unit having a first detector line and a last detector line, the detector lines proceeding substantially at a right angle relative to the rotational axis, wherein a measured dataset allocated to respective rotational angles are registered during a number of periods at a number of rotational angles, simultaneously by each the detector line per rotational angle, wherein the presence of phase ranges are identified with respective phase reference points of the periodic motion of the object, and the measured datasets are registered at least during such phase ranges, wherein the duration of the phase ranges is identified and the product of the number of periods and a phase angle range swept during the phase range corresponds to at least a reconstruction angle range required for the reconstruction of an image of the object, and wherein an image of the object is reconstructed from the image datasets with a three-dimensional back-projection algorithm.
This image reconstruction method embodiment is preferably utilized for imaging the human heart and the phase range lies in the beat phase of the human heart. For determining the phase range, an electrocardiogram of the human heart is again preferably monitored.
The x-ray source can be triggered with the electrocardiogram, so that the object is only transirradiated during the phase ranges, the examined patient is subjected to an especially low x-ray dose.
The rotational speed of the carrier can be selected such that the measured datasets of immediately succeeding rotational angles are registered during the phase range of the same period, or during the phase range of the immediately following periods, so that the image reconstruction is especially simple. When, by contrast, the rotational speed of the carrier is selected as high as possible, the examined patient is subjected to a lower x-ray dose.
construction of an overall measured dataset adequately large for the image reconstruction can, for example, ensue by combining, per phase angle, the measured datasets registered during the phase range are combined per phase range to form a respective rotational angle group, identifying a reference angle corresponding with the phase reference point for each rotational angle group, identifying, per rotational angle, the rotational angle groups whose reference angle is maximally as large as that of the respective rotational angle, and utilizing the measured datasets of that rotational angle group within the rotational angle groups identified in this way for reconstruction of an image of the object, at which the difference between the respective rotational angle and the respective reference angle is minimal.
Alternatively, the overall measured dataset can be constructed by dividing the reconstruction angular range into a number of sub-angle ranges of identical size, each having a respective sub-angle range reference angle, combining, per phase range, the measured datasets registered during the phase range, to form a respective rotational angle group, identifying, for each rotational angle group, a reference angle corresponding to the phase reference point, and utilizing, per sub-angle range, the measured datasets of that rotational angle group at which the absolute value of the difference between the respective sub-angle range reference angle and the respective reference angle is minimum for the reconstruction of an image of the object.
Given a change from one rotational angle group to another rotational angle group, the measured datasets can be weighted and superimposed in an overlap region, so that a higher image quality can be achieved.
When the examination ensues in the form of a spiral scan, the inventive method including obtaining a data from a detector unit having a first detector line and a last detector line, the detector lines proceeding substantially at a right angle to a rotational axis and parallel to the rotational axis and being spaced from one another by a detector width, shifting the object along the rotational axis relative to the carrier with a feed rate and rotating the carrier around the rotational axis with a rotational speed, identifying, per rotational angle, the presence of resting phases and, for each of a number of rotational angles, registering respective measured datasets allocated to the respective rotational angle simultaneously with the detector rows during the resting phases, identifying the duration of the resting phases and selecting the rotational speed such that the carrier rotates during a resting phase through a rotational angle that is at least as large as the reconstruction angular range required for the reconstruction of the object, and selecting the feed rate such that the object is maximally shifted along the rotational axis by the detector height during the sum of a motion phase and two reconstruction times, with the reconstruction time being the time required for sweeping the reconstruction angular range.
In this embodiment, namely, measured datasets can be registered during the resting phases in such a large continuous rotational angle range that, using a known interpolation between the connector lines for each slice or shift position registered within this resting phase, a reconstruction of the object is possible with back-projection algorithms that are well-known in computed tomography. A feed of the object that is not too fast ensues in the motion phases, so that the shift positions registered in the following resting phase merge seamlessly with the previously register shift positions. By combining the measures of a multi-line detector unit, registration of the measured datasets in the resting phases, and suitable selection of feed rate and rotational speed, qualitatively high-grade images of the object thus can be registered and reconstructed with known reconstruction algorithms.
When, by contrast, the periodic motion of the object exhibits no resting phase or only a short resting phase or registrations of the object are to ensue during the motion phase, alternatively for spiral scans the method includes obtaining data from a detector unit having first detector line and a last detector line, the first and the last detector lines proceeding at a right angle relative to the rotational axis and being spaced from one another by a detector width parallel to the rotational axis, shifting the object along the rotational axis relative to the carrier with a feed rate, and rotating the carrier rotates around the rotational axis with a rotational speed, for each a number of rotational angles registering respective measured datasets allocated to the respective rotational angle simultaneously with the detector lines, identifying the presence of phase ranges with respective phase reference points of the periodic motion of the object, and registering the measured datasets at least during such phase ranges, selecting the feed rate such that the object exhibits a number of periods during the feed within the detector width, and identifying the duration of the phase ranges, with the product of the number of periods and a phase angle range swept during the phase range at least corresponding to a reconstruction angular range required for reconstruction of the object.
This measured data registration method embodiment is preferably utilized for imaging the human heart and when the phase range lies in the beating phase of the human heart.
An electrocardiogram of the human heart is again preferably monitored for the determination of the phase range.